As the demand for high-performance integrated circuits continues to grow, the use of materials having high electrical conductivity also increases. The most commonly used material for gate electrodes, capacitor plates, and many types of interconnects is polycrystalline silicon. However, since polycrystalline silicon is a semiconductor material, its intrinsic state is not highly conductive. The electrical conductivity of polycrystalline silicon can be increased by doping the polycrystalline silicon with a conductivity determining dope, such as boron, phosphorus, or arsenic. However, once the polycrystalline silicon is saturated with a dopant, a further increase in electrical conductance can not be obtained. Other materials, such as pure metals, have a very high electrical conductivity, but have not been widely used in metal-oxide-semiconductor (MOS) technology, in part, because they do not form a satisfactory interface with silicon dioxide.
To overcome the interface limitations encountered with pure metals, high-performance integrated circuits requiring very low electrical resistance leads employ an alloy of polycrystalline silicon and a refractory-metal. This material is known as a refractory-metal silicide. The silicide material is fabricated in such a way as to maintain the superior interface characteristics of polycrystalline silicon and silicon dioxide, while achieving low electrical resistance through the incorporation of a refractory-metal.
A common method for fabricating a refractory-metal silicide material is to first form a polycrystalline silicon layer, then deposit a refractory-metal layer onto the polycrystalline silicon layer. The refractory-metal layer can be deposited onto a silicon substrate also. After depositing the refractory-metal layer, the layers are heated to a high temperature and the refractory-metal diffuses into the polycrystalline silicon. The refractory-metal atoms react with silicon atoms to form a refractory-metal silicide. This process is known in the art as a salicide process. A typical process sequence of the prior art for the fabrication of a silicided MOS device is illustrated in FIGS. 1 and 2.
FIG. 1 illustrates, in cross-section, a portion of a semiconductor substrate 10 having already undergone several process steps in accordance with the prior art. A field oxide region 12 defines an active region 14 in a semiconductor substrate 10. A gate electrode.16 of polycrystalline silicon overlies active region 14 and is separated therefrom by gate oxide layer 18. Insulating sidewall spacers 20 and 21 reside adjacent to the edges of gate electrode 16. After fabricating the gate electrode structure and forming the side wall spacers, a layer of refractory-metal 22 is deposited to overlie gate electrode 16, active region 14, and field oxide region 12.
Once the refractory-metal layer is deposited, an annealing process is carried out at a high temperature to form a refractory-metal silicide 24 in gate electrode 16, and a refractory-metal silicide 26 in active region 14. As shown in FIG. 2, the silicide-forming process results in an unevenly distributed silicide region within both gate electrode 16 and substrate 10. The uneven distribution of refractory-metal silicide is a result of a non-uniform diffusion of refractory-metal atoms into both the polycrystalline silicon material of gate electrode 16, and the single crystalline silicon material of substrate 10.
The non-uniform silicide regions, formed by fabrication processes of the prior art, result in a large variance in the electrical conductivity within the silicided regions. The non-uniform electrical conductivity can reduce the performance of an integrated circuit by causing the signal processing speeds to vary between components within the integrated circuit. The processing speed variance can lead to timing problems and result in slow or erratic signal processing within the integrated circuit. Accordingly, improvement in the fabrication of complex integrated circuits having refractory-metal silicide components is necessary to provide reliable, high-performance integrated circuits.